How to measure floating high voltage AC with Op-Amp

Measuring high voltage AC is not safe just with a resistor voltage divider. Or sometimes, we can not connect one terminal to the GND of the measuring circuit. Sometimes, we need to measure a floating high voltage. In this article, we are going to learn how to do that floating high voltage measurement using Op-Amp.

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What is floating voltage?

In most cases, one terminal of the measuring point is connected to the GND of the measuring circuit. But sometimes, we can not do that. Like if you wish to measure 220V AC using your MCU circuit, you should not connect the Neutral terminal to the GND of your MCU circuit to keep it safe. In this case, you need to keep that Neutral terminal floating.

That means, the floating voltage means, the voltage difference between two points of which are not connected to VCC nor GND.

How to measure floating voltage?

There are different ways to measure this floating voltage. You can use a transformer, Op-Amp, or even optocouplers.

Using a transformer will make it safe and isolated but it will increase the size and weight of measuring circuitry. On the other hand, Op-Amp is cheap as well as small in size. Also, provide good linearity. But Op-couplers can give you the status of voltage presence but can not maintain good linearity.

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Circuit diagram using Op-Amp

Here in this article, we’ll make our circuit with Op-Amp to measure high voltage.

As you can see, we used multiple resistors in series with a high voltage source. R1, R3, R4, R5 are connected in series. Using multiple resistors provide good isolation to AC high voltage. Also, heat due to power loss is significantly reduced.

In this circuit, you can see that we used a voltage reference of 2.5V which is feed to the non-inverting terminal through the R10 resistor. Here in this circuit, R9 is working as our feedback resistor.

The gain of this circuit is:

Gain = R9/(R2+R6+R7+R8);

Note that, R1+R3+R4+R5 must be equal to R2+R6+R7+R8. Or distortion in output will arise.

The voltage reference should be selected carefully to obtain the full range of voltage. Note that, the output null point is 2.5V. That means if there is no input voltage, there will be a 2.5V DC at the output terminal.

Simulation result:

Here is the simulation result:

As you can see the result in the simulation. But I’ve been using this circuit in many products for many years. It works fine. You can easily implement this circuit in your circuit too.

I hope you learned something and enjoyed this project. If you need any help, I’m Mithun here for that. Thank you, Enjoy!

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